1. Field of Invention
This invention is concerned with notch filters, and more particularly notch filters capable of operating with a relatively narrow notch or missing frequency band and a relatively large signal attenuation in the notch.
2. Description of the Prior Art
There are TV satellite receiving antennas operating in C band at 3720 megahertz to 4480 megahertz which is the same band at which telephone company long line transmitters operate, creating the probability of signal interference. Such received signals are converted into the UHF band of 270 to 770 megahertz for further handling and processing. For purposes of providing operating power to amplifiers located at the antenna dish a dc (direct current) signal is also present with the UHF signal. Without filtering of the unwanted telephone frequencies the interference is visible on television sets. Therefore, filtering of the unwanted telephone signals is essential. Yet it is also essential that only the telephone frequencies are removed so as not to degrade the television signals. The type of filter wanted is a notch filter capable of removing only a narrow frequency band of 6 MHz (a center frequency f.sub.o .+-.3 MHz) at 3 dB attenuation and a band of about 5 to 10 kilohertz (f.sub.o .+-.2.5 KHz to f.sub.o .+-.5 Khz) at 50 dB attenuation while passing all other frequencies with little or no attenuation.
Notch filters in general are known. Two companies which make commercial versions of notch filters are Blonder-Tongue Laboratories, Inc., One Jake Brown Rd., Old Bridge, N.J. 08857 and RF Systems Division, General Instruments Corporation, 4229 S. Fremont Ave., Tuscon, Ariz. 85714. Such filters operate in the VHF spectrum with an ac only input signal. Notch filters operating in the UHF frequency band capable of passing dc with attenuation of at least fifty decibels and removing only about 5 to 10 kilohertz at 50 dB attenuation are not available.